This application claims Paris Convention priority of DE 100 07 679.3 filed Feb. 19, 2000 the complete disclosure of which is hereby incorporated by reference.
The invention concerns a method of operating an NMR (=nuclear magnetic resonance) spectrometer, in particular a high-resolution NMR spectrometer, comprising a DDS (=direct digital synthesis) generator which contains an NCOL (=numerical controlled oscillator) for generating an LO (=local oscillator) frequency, e.g. the first LO frequency fLO1, wherein the frequency of the NCOL is defined in the DDS generator through input of a numerical value Z.
An NMR spectrometer comprising such a DDS is disclosed in the company leaflet xe2x80x9cAVANCE/Digital NMRxe2x80x9d of Bruker AG, Fxc3xa4llanden, Switzerland, dated March 1999, wherein in particular page 11 shows a functional unit xe2x80x9cDDSxe2x80x9d performing as xe2x80x9cfrequency and phase controlxe2x80x9d in the CPU.
Frequency generators which operate with direct digital frequency synthesis, so-called DDS generators (DDS=Direct Digital Synthesis) are described e.g. in xe2x80x9cFrequency Synthesizers Design Handbookxe2x80x9d, J. A. Crawford, Artech House, Boston, London, 1994, page 346 or in xe2x80x9cDigital PLL Frequency Synthesizers. Theory and Designxe2x80x9d, U. L. Rohde, Prentice-Hall Inc., Englewood Cliffs, N.J. 1983, page 110.
The DDS generators have the following positive characteristics:
They generate numerical values with a clock rate given by an externally supplied constant clock frequency fs, and the numerical values represent a signal of a desired frequency. This signal is subsequently converted in a DAC (=Digital to Analog Converter) into an analog signal which has actually the same frequency stability as the clock frequency and is therefore very stable. The frequency cannot be changed continuously, but only in discrete frequency steps which may be very small using current methods, i.e. in the milliherz range for output frequencies between 10 and 30 MHz thus allowing almost continuous frequency adjustment.
The DDS generators essentially require only digital IC components which keeps their manufacturing costs low. A very advantageous solution consists in that the entire DDS generator is integrated in one single ASIC component (ASIC=Application Specific Integrated Circuit) which can considerably reduce costs when a large number are produced and allows particularly dense packing of the functional digital elements. The latter is particularly advantageous in fast electronic processes which are increasingly required today.
These positive aspects of a DDS generator, however, face the serious drawback that the spectral purity of the output signal is no longer sufficient for today""s standards. DDS generators have been successfully used for more than 10 years in NMR (=nuclear magnetic resonance) spectrometers. The demand for spectral purity of the LO signals has increased in such a way that these generators can no longer provide the high performance needed during the receiving phase of the NMR signal.
The insufficient spectral purity of the DDS generator is caused by the so-called quantizising noise which is due to the fact that the signal generated in the DDS generator is quantizised, i.e. represents a stepped approximation to the desired signal, wherein the numerical values of these steps are defined only with a finite accuracy given by the maximum number of available bits.
The quantizising noise decreases the larger the number of steps within one period and the higher the accuracy of the numerical values of said steps. The number of steps cannot be increased arbitrarily. There is a limit given by the maximum clock rate of the digital components.
NMR signals in high-resolution NMR often consist of very strong and at the same time very weak frequency components, wherein the weak components are frequently the significant ones. This means that the NMR signal has a large dynamic range. One of the most sensitive mixing stages in the NMR receiver is the first mixing stage which uses an LO signal (fLO1) derived from the DDS generator and thus includes quantization noise. If this LO signal is mixed with the NMR signal, the quantization noise will be transferred particularly to the strongest frequency components of the NMR signal and will thus generate in the NMR spectrum a base line disturbed by unwanted frequency components. This disturbed base line also includes the desired weak frequency components of the NMR signal which are difficult to distinguish from the disturbing components. As a result, proper spectroscopy is impossible.
During the relatively uncritical transmitting phase in NMR spectroscopy, DDS generators are still used today without any problems.
However, during the critical receiving phase, the demand for spectral purity is very high today such that the DDS generator which provides the variable LO frequency does no longer meet these demands due to the quantization noise described above. Up to now, no practicable method has been available to reduce said quantization noise. Therefore, in all critical experiments which required high spectral purity, one had only the choice to do without this elegant and powerful generator or accept its disadvantages.
It is therefore the underlying purpose of the present invention to present a method comprising the initially mentioned features utilizing a DDS generator even when very high spectral purity is required, wherein particularly the quantization noise is eliminated as much as possible in the frequency range of the NMR spectrum.
In accordance with the invention, this object is achieved in a simple and effective way in that the numerical value Z is selected such that it assumes only values which satisty the equation
Z=nxc2x7N/m
wherein Z, n, N, and m are positive integers, wherein N is a power of 2 with a positive integer exponent, said exponent representing the maximum number of bits during the calculation process, wherein m is approximately 2xc2x7fs/xcex94B, n is approximately mxc2x7fout/fs and m a common integer divisor of nxc2x7N and wherein fs is the clock frequency of the NCOL, xcex94B the desired bandwidth with high spectral purity and fout the output frequency of the NCOL.
According to the inventive teaching, it is not allowed to use arbitrary but only selected Z values for the input to the DDS generator. As a result, the lowest occurring disturbing frequency will always be larger than the repetition frequency xcex94fRaster at which the signal of the NCOL repeats itself exactly. In this way it is possible to select the above described disturbing components with a sufficient separation to ensure that the NMR spectrum in between remains undisturbed.
In a variant of the inventive method which is particularly easy to carry out and is thus used with particular preference, m is a power of 2 having a positive integer exponent. This considerably simplifies the calculations to be carried out in the inventive method with respect to the general case and as a result the amount of calculations needed is reduced considerably.
The method is particularly facilitated in a further development of the above-mentioned variant, wherein the calculation of Z is carried out in the following three stages.
(a) the value for m is determined by means of the equation
m=2RndDwn{log[2fs/xcex94B)/log 2]}
wherein xcex94B is the desired bandwidth of high spectral purity, fs is the clock frequency of the NCO and RndDwn is a rounding-off process to the next smaller integer value;
(b) the value for n is determined through equation
n=Rnd(mxc2x7fout/fs)
wherein fout is the desired frequency of the NCO, m the value calculated in the first stage and Rnd a rounding-off process to the next integer value;
(c) the value for Z is determined through equation
Z=nxc2x7N/m
wherein N is defined in claim 1 and m and n are the values determined in stages (a) and (b).
The present invention also includes a DDS (=Direct Digital Synthesis) generator for application in NMR spectrometers, in particular high-resolution NMR spectrometers comprising an NCOL (=Numerical Controlled Oscillator) for generating an LO (=Local Oscillator) frequency FLO1 which is characterized in that the DDS generator contains several NCOs for generating a transmitting frequency. If at least two NCOs are present, one can be utilized for supplying the phase information for detecting the FID signal and the other one for changing the transmitting frequency during the transmitting phase.
A preferred embodiment of the inventive DDS generator is characterized in that the NCOL has a clock frequency fs which meets the condition fs=2kxc2x7f0 wherein k is a positive integer and f0 is the base frequency from which all LO frequencies, except for two, namely fLO1 for a mixing stage and the LO frequency for a DQD (=Digital Quadrature Detector) are derived such that they are an integer multiple of f0. As a result, the disturbing components generated in the subsequent mixing stages are identical with the grid components of the NCOL.
One further development of the invention is particularly preferred wherein one of the NCOs oscillates continuously and can provide a reference phase for all other NCOs by transferring its actual phase to the other NCOs via switches thereby achieving an exact definition of the initial phase of the FID signal and allowing phase synchronism for several successive FID signals.
A further particularly preferred embodiment of the inventive DDS generator is characterized in that a saw tooth to sinusoidal signal transformer is provided for transforming the saw tooth signal of an NCO into a sinusoidal signal and in that a further saw tooth to cosine signal transformer is provided for transforming the saw tooth signal of this NCO into a cosine signal thereby producing two channels which are in quadrature with one another and can be used in a subsequent frequency synthesizer for a quadrature mixing stage. A quadrature mixing stage produces considerably less undesired mixing components compared to a normal mixing stage.
One embodiment is also preferred which preferably comprises digital multiplicators which are fed with signals from signal transformers and where the desired amplitude dependence is achieved by a numerical calculation process during the transmitting phase. In this way, a digital amplitude modulator can be produced with simple means which has a much higher precision than an analog modulator.
One further embodiment of the inventive DDS generator is also preferred which comprises an attenuator whose phase and attenuation errors can be compensated in that the phase errors are stored as a function of the desired attenuation value in a first memory and that the attenuation errors are stored as a function of the desired attenuation value in a further memory and in that during setting of a desired attenuation value, the associated phase error is added with reverse signs to the current signal in one adding stage, and the corresponding attenuation error with reverse signs is added to the desired attenuation value and supplied to the attenuator. Registration of the attenuation errors can thus allow mathematical pre-compensation of the signals thereby obtaining the desired attenuation values practically without phase and attenuation errors.
One method is also advantageous for operating an inventive DDS generator with DQD which is characterized in that during the receiving phase, exact positioning of the NMR spectrum in the low frequency range is not effected via NCOL but by means of the numerical value ZQ in the DQD. This allows fine adjustment of the NMR spectrum to a desired frequency range without generating additional disturbing components.
Further advantages of the invention can be extracted from the description and the drawing. The features mentioned above and below can be utilized in accordance with the invention either individually or collectively in any arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but rather have exemplary character for describing the invention.
The invention is shown in the drawing and further explained by means of embodiments.